I. FIELD OF THE INVENTION
This invention relates to the field of Raman spectroscopy and to lasers for Raman excitation.
II. DESCRIPTION OF THE PRIOR ART
Sample fluorescence hinders Raman spectroscopy applications (especially those which use visible laser excitation) by masking the Raman signal and making vibrational characterization difficult if not impossible. A common way to avoid this problem is with the use of NIR laser excitation (C. J. Frank, D. C. Redd, T. S. Gansler, and R. L. McCreery, Anal. Chem., 66, 319 (1994)). NIR laser excitation has the additional advantage of high transmittance through optical fibers which can be used for remote spectroscopy. Traditionally, the sources for such excitation in the laboratory have been either dye lasers or solid state lasers, both of which are large, expensive, have limited lifetimes, and have high operating costs.
More recently, laboratories have begun to report the use of diode lasers as the excitation source (R. L. McCreery, Proc. SPIE-Int. Soc. Opt. Eng., 1637 (Envir. Process Monit. Technol.), 208 (1992); C. D. Allred and R. L. McCreery, Appl. Spectrosc., 44, 1229 (1990); C. H. Tseng, C. K. Mann, and T. J. Vickers, Appl. Spectrosc., 47, 1767 (1993); S. M. Angel, T. J. Kulp, and T. M. Vess, Appl. Spectrosc., 46, 1085 (1992); J. B. Cooper, J. Aust, C. Stellman, K. Chike, and M. L. Myrick, Appl. Spectrosc., 50, 567 (1994); J. M. Williamson, R. J. Bowling, and R. L. McCreery, Appl. Spectrosc., 43, 372 (1989); C. J. Frank, D. C. Redd, T. S. Gansler, and R. L. McCreery, Anal. Chem., 66, 319 (1994); S. M. Angel, M. L. Myrick, and T. M. Vess, Proc. SPIE-Int. Soc. Opt. Eng., 1435 (Opt. Methods Ultrasensitive Detect. Anal.: Tech. Appl.), 72 (1991); N. Yamamoto, W. Z. Huang, and H. Horinaka, Jpn. J. Appl. Phys., Part 1, 32 (Suppl. 32-3, Proceedings of the 9th International Conference on Ternary and Multinary Compounds, 1993), 123 (1993); H. Horinaka and N. Yamamoto, Jasco Rep., 34, 1 (1992); H. Horinaka, N. Yamamoto, and H. Hamaguchi, Appl. Spectrosc., 46, 379 (1992); Y. Wang and R. L. McCreery, Anal. Chem., 61, 2647 (1989); S. M. Angel and M. L. Myrick, Anal. Chem., 61, 1648 (1989); A. Rubens, B. De Castro, and P. R. B. Pedreira, Opt. Commun., 62, 348 (1987)). These lasers offer single mode NIR output (780-860 nm) at relatively high optical power (150 mW), they are compact (the laser itself is .about.1.3 cm in diameter and 2 cm thick, the heat sink is .about.10.times.5.times.5 cm, and the power supply and temperature controller unit is .about.30.times.30.times.13 cm), and they are inexpensive ($1000-$2000 for laser and heat sink and $1000-$5000 for controller). In addition, lifetimes are typically two to three orders of magnitude greater than conventional lasers. Diode laser Raman investigations reported in the literature have been confined to the use of Fabry-Perot type (index guided) lasers. In this type of laser, an active region of GaAlAs is "sandwiched" between a p-doped layer of GaAlAs and an n-doped layer of GaAlAs. When a voltage is applied across this sandwich structure, current is injected and subsequent electron/hole pair re-combination in the active region give rise to photon emission. Above the current threshold (typically 30 mA) population inversion is achieved giving the resultant laser action. The higher index of refraction of the active region relative to the doped regions act to constrain the emitted photons to the active region thus forming the cavity for the laser, hence the common name "index guided" laser.
III. PROBLEMS PRESENTED BY PRIOR ART
For an index guided diode laser, the frequency of the emitted photons is totally dependent on the band gap of the semiconductor device, and it is this dependence which gives rise to many of the problems associated with diode laser sources for spectroscopy. Since the band gap is dependent on the temperature of the device, changes in temperature result in changes in laser wavelength. For spectroscopic applications, a thermoelectric cooler is thus required for the laser. Typically, the laser wavelength shifts 0.3 nm with every 1.degree. C. change in temperature. Unfortunately, the change in band gap with temperature is often plagued with hysteresis so that when a laser is shut down or the temperature is changed from initial conditions, reproducing the initial conditions does not necessarily reproduce the initial wavelength. Under such conditions, Raman spectra acquired at a set spectrograph grating position will have an apparent shift when compared to previously acquired spectra at the same temperature.
The band gap is also dependent on the injection current. As the current level is changed in order to increase or decrease the optical power output, regions of instability are often encountered where the laser wavelength will shift, or will emit multiple wavelengths or even oscillate between two wavelengths at an undetermined rate. These events are often referred to as "mode hops". Even when a region of stability (with regards to the current and the optical output frequency) is attained, there is no guarantee that the device will remain stable under these conditions at a later date after some change in conditions has occurred. Finally, the optical output also can be destabilized by photons which are back reflected into the laser by collimating optics, pigtails, etc. (also referred to as optical feedback). The net result is a laser which gives highly ir-reproducible behavior over the course of time, thus limiting its use in any type of industrial control application.
A recent improvement in diode lasers has come with the development of external cavity diode lasers. In this type of laser, an external grating is used to provide selective feedback to the active region so that lasing only occurs at one wavelength, the remaining emission wavelengths are dispersed outside of the cavity by the grating. By changing the position of the grating, the laser wavelength can be tuned to a desired value. Typically the tuning range is 20 nm with a laser line width of 1-5 MHz. The selective feedback also eliminates hysteresis and mode hopping. Unfortunately, these lasers remain expensive, typically costing &gt;$20,000. Additionally, the external grating requires the laser to be larger and less robust than conventional diode lasers.